1. Field of the Invention
The present invention pertains to a water jet propulsion assembly and, more particularly, a stationary vane arrangement for directing the pressurized flow of water from an impeller of a water jet propulsion unit.
2. Discussion of the Prior Art
A jet ski-type watercraft utilizes a water jet propulsion unit located in a concave area formed in a lower, stern portion of the watercraft to propel the watercraft. As shown in FIG. 1, a typical prior art water jet propulsion unit draws water in through an opening 38A formed in the hull of the boat. This water is pressurized by an impeller 30A which is rotated by an engine (not shown) through a shaft 16A. The pressurized water is then expelled rearward through a nozzle 40A. A duct structure 26A defined by an outer duct tube 20A and an inner duct tube 24A defines a water passage 36A for the pressurized water flowing toward nozzle 40A. Stationary vanes 23A, formed integral with duct 26A and extending radially inward therefrom, function to convert the swirling water flow from impeller 30A to a linear flow and to direct the water to nozzle 40A.
In order for stationary vanes 23A to be able to convert the swirling water flow into a linear flow, stationary vanes 23A must be colinear with the axial direction of drive shaft 16A. As best shown in FIG. 3, which is a cross-sectional view taken along line III--III of the stationary vane 23A shown in FIG. 2, each stationary vane 23A comprises a sloping portion 50A and a straight portion 52A. Sloping portion 50A comprising roughly half the length of vane 23A, is located on the impeller side of duct 26A and slopes in a direction opposite that of impeller 30A. Straight portion 52A of stationary vane 23A extends axially toward the nozzle end of duct 26A. This type of duct arrangement for a jet ski-type watercraft is exemplified by Japan Patent Sho 60-30599.
In this known jet ski propulsion arrangement, duct 26 A, along with stationary vanes 23A, is produced by aluminum die casting in order to improve productivity and to reduce manufacturing costs. However, since the stationary vanes 23A have sloping portions 50A, the fabrication has to be accomplished using a split mold, as depicted at 80A, 82A in FIG. 3, in order to cast duct 26A having stationary vanes 23A. This makes it necessary for each of the mold pieces to be removed in the directions of arrows 70A, 72A of FIG. 3. However, when stationary vanes 23A are made relatively long, the mold removal distance lengthens correspondingly. When this is done, as shown in FIGS. 2 and 4, a step 60B is formed on the outside circumference of the inner duct tube 24A along with a step 60A on the inside circumference of outer duct tube 20A. These steps 60A, 60B formed at the junction of the mold sections tend to disturb the water flow by creating turbulents and decrease the propulsion efficiency of the water jet propulsion unit.
Therefore, there exists a need in the art for a water jet propulsion assembly which can be die casted with minimal step differentials and which includes a water flow duct having radial vanes which enables a generally linear flow of water through the duct so as to increase the efficiency of the propulsion assembly.